Nail Hematoma

A finger slammed in a door, a thumb hit with a hammer, a brick dropped on a toe — these nail injuries are common and extremely painful. They can cause blood to pool visibly under a nail, which is called a subungual hematoma.

A subungual hematoma occurs when an injury breaks open blood vessels under the nail, causing blood to collect and become trapped in one spot.

Poorly fitting shoes can also cause subungual hematomas, especially if the person is very active. Shoes that are too tight or narrow can put pressure on the toenail, breaking blood vessels and leading to a pool of blood under the nail.